Method of growing gallium nitride crystal and gallium nitride substrate

ABSTRACT

The GaN facet growth method produces defect accumulating regions H on masks by forming a dotmask or a stripemask on an undersubstrate, growing GaN in a reaction furnace in vapor phase, inducing GaN crystals on exposed parts without covering the masks, inviting facets starting from verges of the masks and producing defect accumulating regions H on the mask. The defect accumulating regions H have four versions, that is, non (O), polycrystal (P), c-axis inclining single crystal (A) and orientation inversion (J). The best is the orientation inversion region (J). A sign of occurrence of the orientation inversion regions (J) is beaks of inversion orientation appearing on facets. GaN is grown on a masked undersubstrate by supplying a carbon material at a hydrocarbon partial pressure of 10 Pa to 5 kPa for 0.5 hour to 2 hour by an HVPE facet growth method without burying facets.

RELATED APPLICATION

This application is a Continuation-In-Part (CIP) of application Ser. No.10/933,291 filed on Sep. 3, 2004 and application Ser. No. 10/936,512filed on Sep. 9, 2004.

This application claims priority to Japanese Patent ApplicationNo.159880/2006 filed on Jun. 8, 2006.

FIELD OF THE INVENTION

The next generation large capacity photodiscs make use of GaN typeblue/violet lasers. Practical production of the GaN type blue/violetlasers requires high quality GaN substrates. This invention relates to amethod of growing GaN crystals in order to produce high quality GaNsubstrates.

GaN type semiconductor lasers of a 405 nm wavelength are promised to beused for reading out the high density photodiscs. At present,blue/violet LEDs (Light Emitting Diodes) are produced by piling GaN,InGaN, etc. films on sapphire (Al₂O₃) substrates (wafers). Sapphire israther different from GaN in lattice constant. Lattice misfit induceshigh density defects in the GaN, InGaN films piled on sapphire wafers.Current density is small in LEDs. Due to the small current density,defects are not proliferated in LEDs. On-sapphire GaN type LEDs areprevalent. Current density is far large in laser diodes (LDs).

The large current density proliferates dislocations in LDs. Sapphiresubstrates are inappropriate for LDs, because high current densitymultiplicates dislocations and degenerates the LDs. Unlike LEDs,on-sapphire blue/violet GaN LDs have not been put into practice.

There is no material which has a lattice constant sufficiently close toGaN. It turns out that the substrates on which GaN films are safelygrown should be GaN substrates. Realization of GaN type blue/violetlasers ardently requires high quality GaN substrates with lowdislocation density.

Crystal growth of GaN is very difficult. Heating GaN does not make amelt of GaN. No GaN melt can be obtained. Conventional liquid phasecrystal growth methods are inapplicable to GaN crystal growth. Manytrials have been done for growing GaN substrate crystals from vaporphase by supplying gaseous materials. Various attempts have been donefor growing GaN crystals in order to produce practically large sized GaNsubstrate wafers.

The inventors of the present invention have contrived a method offorming masks on a foreign material undersubstrate, growing a GaN filmcrystal on the masked undersubstrate, eliminating the undersubstrate andobtaining a GaN freestanding crystal.

(1) WO99/23693 which was invented by the inventors of the presentinvention proposed a method of forming a stripe mask with parallelstripes or a dot mask with uniformly distributing isolated round dots ona GaAs undersubstrate, growing a thick GaN crystal on the mask-coveredGaAs undersubstrate, removing the GaAs undersubstrate and obtaining afreestanding GaN crystal substrate. The masks have wider covered partsand narrower exposed parts (windows, holes). Namely the masks arecovered part prevailing masks. GaN crystal nuclei happen only on exposedparts at the initial stage. Masks prevent nuclei from occurring. GaNnuclei dilate, unite , make films and produce cones on the exposedparts. GaN grains overstep the window margins onto masks. GaN crystalsgrow on the masks in horizontal directions. Dislocations expand also inthe horizontal directions on the masks. Then horizontally expandingcrystals collide together on the masks. Then the direction of growthchanges from the horizontal direction to the vertical direction. After.the collision, the GaN crystals grow upward. Dislocations turn thedirection. Dislocations expand in the vertical direction. Twice changesof extending directions reduce dislocations at an early stage of growth.Then GaN grows in the vertical direction with a C-plane surface.

The method capable of decreasing dislocations by changing the directionsof growth twice is called an Epitaxial Lateral Overgrowth (ELO) method.A freestanding GaN crystal enjoying a low dislocation density isobtained by eliminating the GaAs undersubstrate. (1) WO99/23693 proposeda method of growing a thick GaN crystal on the obtained GaNundersubstrate in vapor phase, slicing the thick GaN crystal severaltimes in the planes vertical to the growth direction and obtaining aplurality of GaN wafers. There are an MOCVD method, an MOC method, anHVPE method and a sublimation method for vapor phase growth methods ofGaN. (1) WO99/23693 advocated the HVPE (Hydride Vapor Phase Epitaxy)method because the HVPE has an advantage of a high growing speed.Considerably high density of dislocations accompany the GaN crystalsmade by the ELO/HVPE method. The GaN crystals produced by (1) WO99/23693based on the ELO/HVPE are high defect density crystals of low quality.When GaN type photodevices are made on a high defect density low qualityGaN substrate, the devices are also bad, malfunctioning photodevices.Production of high quality photodevices requires low defect density highquality GaN substrates. In particular, mass production of GaNphotodevices demands high quality GaN wafers with low dislocationdensity in wide regions. (2) Japanese Patent Laying Open No.2001-102307which was invented by the inventor of the present invention proposed anew method of GaN growth effective in reducing dislocations on GaNsubstrates.

The method proposed by (2) Japanese Patent Laying Open No.2001-102307decreases dislocations by growing a thick GaN crystal, gatheringdislocations from other regions into definite regions and loweringdislocation density at the regions other than the definite regions.

(2) Japanese Patent Laying Open No.2001-102307 mentioned a method ofgrowing a GaN crystal, forming a three dimensional facet structure, forexample, inverse-hexagon cone pits consisting of facets in the growingGaN crystal, maintaining the facet structure without burying the facetstill the end of growth, gathering dislocations from other regions intothe facet pits and lowering the defect density in other regions. FIG.1(a) and FIG. 1(b) show a section of a GaN crystal 4 having a facet pit5 shaped into an inverse hexagonal cone. The surface of the GaN crystalis not an entire smooth surface but a flat surface and facet pits 5distributing on the flat surface. The flat surface 7 is a C-plane. Thepits 5 are sometimes hexagonal cones and at other times dodecagonalcones. In a hexagonal pit, facets meets with neighboring ones at 120degrees. Neighboring facets 6 and 6 are in contact with each other at aboundary 8. The boundaries 8 converge at a pit bottom 9. Bottom ends offacets 6 assemble at the pit bottom 9 which is a converging point of theboundaries 8.

Crystal growth progresses in the directions of normals which are halflines vertical to the planes. An average growth direction is an upwarddirection along a c-axis. The flat C-plane 7 grows in the upwarddirection (c-axis direction). Facets grow in slanting directions normalto the facets. Namely the growth direction of a facet 6 is the directionof the normal standing on the facet 6. An inclination angle of a facetto the C-plane is denoted by “Θ”. The facet growth mode does not buryfacet pits. The growth without burying pits means that the upper C-planegrowth speed u is different from the facet growth speed v. Maintenanceof the constant facet pits requires anisotropic growth speeds indicatedby v=u cos Θ. The facet growth speed v is slower than the upper C-planegrowth speed u.

Dislocations D extend in parallel to the growing direction. Dislocationswhich exist on a facet move toward the nearest boundary with the crystalgrowth. v<u. The facet growth is slower than the C-plane growth inspeed. The extension speed of a dislocation is equal to the growthspeed. The dislocation which once reaches the boundary 8 is fixed to theboundary 8 afterward. Since v<u, the dislocation descends along theboundary 8 and arrives at the pit bottom 9. Dislocations on the facetsare assembled via boundaries to the bottom 9 by the facet growth. Thusplanar defect assembling regions 10 are produced just below theboundaries 8 by the dislocations reaching the boundaries, as shown inFIG. 1(b). Linear defect assembling bundles 11 are made just below thebottom 9 by the dislocations arriving at the boundaries and falling tothe bottom 9. Since facets initially lying on the facets are gatheredinto the planar defect assembling regions 10 and the linear defectassembling bundles 11, the facets 6 become nearly free fromdislocations. Since the facets are low dislocation density, dislocationslying on the C-planes 7 are pulled into the facets 6. The dislocations Don the facets are moved to the boundaries 8 by the facet growth. Whenthe pits exist in high density, dislocations are swept and gathered tothe planar defect assembling regions 10 below the boundaries 8 and thelinear defect assembling bundles 11 below the bottom 9. Dislocations inother regions are reduced. Keeping the facets pits without burying pits5 enables the facets pit to maintain the dislocation reduction effect tothe last of growth.

FIG. 2 is a plan view of a pit for showing the dislocation reductioneffect of facet growth method. The crystal growing direction on a facet6 is parallel to the normal standing on the facet 6 when the facet 6 ismaintained. Dislocations lying on the facet 6 expand also in the normaldirection parallel with the growth direction. FIG. 2 demonstratesdislocations D moving in the same direction as the growth direction onthe facets 6. FIG. 2 shows growth directions and dislocation extensionsas projections on a horizontal plane. In the plan view dislocations movein the direction of the inclination of the facet 6. As the GaN crystalgrows, dislocations D come closer to boundaries 8 and arrive at theboundaries. When the dislocations D reach the boundaries, thedislocations turn the extension direction and move inward along theboundaries 8. An inward movement denotes a relative downward movement inthe GaN crystal growing on the facets. In reality, dislocations extendnot downward but upward. Since v<u, dislocations relatively sink down incomparison with a faster growing GaN crystal. Some dislocations D, whichhave failed in arriving at the bottom, make planar defect assemblies 10below the boundaries 8. Other dislocations D, which have arrived at thebottom 9 of the pit 5, make linear defect assembling bundles 11 belowthe bottom 9. Since dislocations are gathered to the defect assemblies10 and 11, other parts become low defect density.

However, problems have been found in the facet growth method capable ofreducing dislocations by making use of the facet growth.

Problem (1): When the crystal grows thicker and thicker and dislocationsD are gathered more and more at the defect assembling bundles 11, thedislocations D are liable to disperse from the bundles 11. Release ofdislocations makes hazy dispersion around the defect assembling bundles11. Release of dislocations is explained by referring to FIGS. 3(1) and3(2). FIG. 3(1) shows a section of a slim linear defect assemblingbundle 11 formed at a bottom 9 of a pit 5 at an early stage of growth.FIG. 3(2) shows hazy dispersion 13 of dislocations releasing from thelinear defect assembling bundle 11. Occurrence of the hazy dispersion 13signifies a poor power of the linear defect assembling bundle 11 forarresting dislocations D.

Problem (2): The places of the linear defect assembling bundles 11 areaccidentally determined. The linear defect assembling bundles 11distribute at random. The places of the bundles 11 cannot bepredetermined. To say other words, the places of defect assemblingbundles 11 are uncontrollable.

Problem (2) derives from the fact that facet pits happen at arbitrarypoints accidentally determined and facet pits distribute at random. Itis preferable to determine the positions generating facet pits beforethe growth. Problem (1) should be conquered by building barriers forpreventing once gathered dislocations from dispersing again.

The inventors of the present invention have made the followingcontrivance for solving the two problems (1) and (2).

The inventors have noticed that gathered dislocations stay temporarilyin the bundles at the bottom 9 of the hexagonal cone pits 5, thedislocations do not perish and dislocations have a tendency of releasingfrom the bundles and the hazy dispersion 13 of once arresteddislocations occurs.

The inventors have hit on a new idea of adding anannihilating/accumulating device to the defect accumulating bundles.FIG. 4(1) and FIG. 4(2) demonstrate the annihilating/accumulating deviceproduced at the bottom of a facet pit. Masks 23 which are made of amaterial capable of prohibiting GaN from epitaxial growing are formed ina regular distribution on an undersubstrate 21. At the initial stage, nonuclei occur on the masked parts and crystal growth begins only onexposed parts. GaN crystals 24 having a C-plane surface 27 grow on theexposed parts.

The masks 23 prevent GaN from growing. Crystal growth on the masks 23delays. Crystal grows on the exposed parts, leaving the masks uncovered.Facets 26 starting from edges of the masks 23 and pits 25 consisting ofthe facets 26 are produced. The facets 26 and the pits 25 are not buriedbut are maintained until the end of the crystal growth. Crystal growthsweeps dislocations along the facets 26 and guides dislocations into pitbottoms 29. The bottoms 29 of the pits coincide with the masks.Dislocations D are gathered above the masks 23 below the pit bottoms 29.On-mask regions gathering dislocations become defect accumulatingregions H, which follow the bottoms 29 of the pits 25. A defectaccumulating region H consists of a grain boundary K and a core S.H=S+K. The facet growth method succeeds in producing defect accumulatingregions H enclosed by the grain boundary K as a dislocationannihilation/accommodation device by preparing masks on anundersubstrate. The mask 23, the defect accumulating regions H and thepit bottom 29 align in the vertical direction. The masks 23 determinethe positions of defect accumulating regions H and pit bottoms 29. Theportions below the facets on exposed parts are low defect single crystalregions Z. The portions below the C-plane on the exposed parts arecalled “C-plane growth regions Y.”

Dislocations assemble on the defect accumulating regions H. Each of thedefect accumulating regions H has a definite width and is enclosed by agrain boundary K. The boundaries K prevent dislocations from releasingout of the defect accumulating regions H. The grain boundaries K have afunction of annihilating dislocations. The core S is an inner partencapsulated by the boundary K. The core S has a function ofaccommodating and annihilating dislocations. Thus the defectaccumulating region H composed of S and K becomes a dislocationannihilating/accommodating device. It is important to build up thedefect accumulating regions H composed of grain boundaries K and cores Shaving the annihilating/accommodating function by masks. Progress ofcrystal growth changes the section of the facet growing crystal fromFIG. 4(1) to FIG. 4.(2). Dislocations are not released from H, sincedislocations are firmly arrested in H. The same state is kept until theend of the growth. Hazy dispersion of dislocations is forbidden. Thefacet growth solves the difficulty with the hazy dispersion ofdislocations as explained in FIG. 3(2). The substance of the defectaccumulating region H was not fully understood at the beginning of thefacet growth. The defect accumulating region H has no constant structurebut different structures. One defect accumulating region is apolycrystal P and another defect accumulating region H is a c-axisslanting single crystal A. Another defect accumulating region H is ac-axis inversion single crystal J. Sometimes no defect accumulatingregion occurs (O). What structure the defect accumulating region takesdepends upon growth conditions.

The best candidate among O, P, A, and J is the c-axis inversion singlecrystal J which has a c-axis ([0001] direction) inverse to thesurrounding regions Z and Y. In the case (H=J), the defect accumulatingregion H has a polarity inverse to the surrounding crystal Z. A definiteclear grain boundary K is generated around the defect accumulatingregion H.

The grain boundary K has a strong function of attracting, annihilatingand arresting dislocations.

When a defect accumulating region H becomes a polycrystal P or a c-axisinclining single crystal A, a clear definite grain boundary K does notoccur. The defect accumulating region with A or P has a weak,insufficient power of annihilating and accommodating dislocations.

The surrounding regions are classified into two different regions. Apart grown under a facet on an exposed part is called a “low defectdensity single crystal region Z”.

Another part grown under a C-plane on an exposed part is called a“C-plane growth region Y”. Both Z and Y are low defect density singlecrystals having the same orientation. But Z and Y have differentelectric properties. The C-plane growth regions Y have higherconductivity. The low defect density single crystal regions Z have lowerconductivity.

The low defect density single crystals Z and the C-plane growth regionsY are single crystals with an upward c-axis ([0001]). The defectaccumulating regions H of the polarity inversion regions J are singlecrystals with a downward c-axis ([000-1]). Since the orientation isinverse, a continual grain boundary K stably occurs between H and Z. Thegrain boundary K has a strong power of annihilating and arrestingdislocations. Occurrence of K between H and Z is a profitable property.An inner core S and an outer space are definitely discerned by the grainboundary K.

The inversion c-axis region J as a defect accumulating region H is themost effective in lowering dislocation density.

The growth speed of the orientation inversion defect accumulatingregions (H, J) is lower than Z and Y. The orientation inversion defectaccumulating regions H make pits or valleys. Therefore the defectaccumulating regions H can stably stay at the bottoms of pits orgrooves.

The grain boundaries K around the defect accumulating region Heffectively arrest and annihilate dislocations and prohibitsonce-arrested dislocations from escaping. No hazy dispersion ofdislocations occurs. A low defect density GaN substrate crystal withdislocations arrested in the defect accumulating regions H is obtained.

It is possible to produce and fix defect accumulating regions H atarbitrary spots on an undersubstrate with a mask. Defect accumulatingregions H do not occur accidentally and randomly but are generated onpredetermined spots or lines on an undersubstrate. The predeterminedspots/lines mean the masked spots/lines. The property enables the facetgrowth to make high quality GaN crystals with regularly aligning defectaccumulating regions H on predetermined spots or lines.

There are several different shapes of the defect accumulating region H.For example, isolated dot-like defect accumulating regions can beproduced. Stripe parallel defect accumulating regions can be also made.Another shape of defect accumulating regions can be prepared. (3)Japanese Patent Laying Open No.2003-165799 demonstrates regularlydistributing isolated defect accumulating regions. FIG. 10(1) is a planview for showing an example of a dotmask which is composed of regularlyaligning isolated mask dots M on an undersubstrate U. low defect densityGaN crystals are grown on exposed parts. The mask dots M make defectaccumulating regions H thereon. Facet pits having a bottom composed of adefect accumulating region H are generated in a growing GaN crystal.Parts under the facets on exposed parts become low defect density singlecrystal regions Z. Other parts out of the facets on exposed parts becomeC-plane growth regions Y. FIG. 6(2) is a perspective view of a portionof a GaN crystal grown on a dotmasked undersubstrate U. The GaN crystalhas a wide flat C-plane growth region Y. Many polygonal pits composed offacets F occur. The pit bottoms exist just above the mask dots. FIG.10.(2) is a plan view of a flat GaN substrate crystal which is made byeliminating the undersubstrate from the GaN crystal grown on a dotmaskedundersubstrate (M2U), obtaining a freestanding as-cut GaN wafer,grinding the GaN as-cut wafer and polishing the as-ground GaN wafer intoa GaN mirror wafer. On-mask parts become defect accumulating regions H.Low defect density single crystal regions Z and C-growth region Yenclose the defect accumulating regions H. A concentric structure (YZH)appears on the dotmask-grown GaN substrate crystal.

Another alternative of masks is a stripemask having a plurality ofparallel mask stripes. Use of a stripemask can make stripe distributingdefect accumulating regions H in GaN crystals. (4) Japanese PatentLaying Open No.2003-183100 proposed a GaN crystal havingstripe-distributing defect accumulating regions H. FIG. 8 (1) shows anexample of a stripemask pattern. Many parallel mask stripes with a widths are aligned at a pitch p on an undersubstrate U. An exposed part has awidth (p-s). A facet growth method grows a GaN crystal on thestripemasked undersubstrate. FIG. 6(1) exhibits roof-shaped GaN crystalsgrown on the stripemasked undersubstrate. Parallel mountains composed oflow defect density single crystal regions Z are produced upon exposedparts. Slopes of the mountains are facets F. V-valleys appear above themask stripes M. Defect accumulating regions H are produced upon thestripes M. Bottoms of the V-valleys correspond to the stripes M. NoC-plane growth regions Y appear in the example of FIG. 6(1). Otherwise,C-plane growth regions Y appear under flat C-planes on exposed parts. Aflat smooth GaN substrate can be obtained by eliminating theundersubstrate from the roof-shaped GaN crystal of FIG. 6(1), grindingthe roof-shaped GaN crystal into a flat GaN wafer and polishing the GaNwafer. FIG. 8(2) is a plan view of the polished GaN wafer. The GaNsubstrate with Ys has a parallel HZYZHZYZ . . . structure. The GaNsubstrate without Y has a parallel HZHZHZ . . . structure.

FIG. 5(1), FIG. 5(2) and FIG. 5(3) demonstrate steps of a facet growthmethod using a stripemask. FIG. 5(1) shows a part of an undersubstrate Uon which a stripe mask consisting of plenty of parallel mask stripes Mis formed. The mask stripes extend in the direction vertical to thesheet. Parts covered with the stripes are called “masked parts”. Otherparts not covered with the stripes are called “exposed parts”. Theexposed part and the masked part have different functions. GaN is grownon the masked undersubstrate (MU) in vapor phase. Exposed parts allowGaN to make GaN nuclei and grow GaN crystals thereon. Masked partsprohibit GaN from making nuclei at the initial step. GaN crystals havinga C-plane surface are grown on the exposed parts (FIG. 5(2)). The maskedparts are still uncovered. Extension of GaN crystals is stopped at thesides of the mask stripes M. Slants of crystals are produced near theverges of the masks M. The slants are facets F. FIG. 5(2) exhibits avacant stripe M, GaN crystals covering the exposed parts and facets Fstarting from the sides to the top of the GaN crystals.

At a later stage, GaN crystals are grown also above the masked parts.The above-mask space has a cavity because the growth on the masks isdelayed. The crystals on the masks are defect accumulating regions Hconsisting of inversion c-axis crystals. Milder inclining facets F′ andF′ appear on the top of the defect accumulating regions h. Theinclination of F′ is identical to the inclination of the upper slants ofthe beaks Q in FIG. 7(3) and FIG. 7(4). Since the defect accumulatingregions H grow upon unified beaks Q, the orientation of the defectaccumulating regions H is identical to the beaks Q. GaN crystalsfollowing the facets F on exposed parts are low defect density andsingle crystals. Thus the portions under the facets on the exposed partsare named low defect density single crystal regions Z. The otherportions growing under the C-plane surface are also low defect densityand single crystals. The portions are named C-plane growth regions Y fordiscriminating from Z. Grain boundaries K are formed between the defectaccumulating regions H and the low defect density single crystal regionsZ. The boundaries between different inclination angle facets F and F′coincide with the grain boundaries K.

In the stripemask case having plural parallel mask stripes, defectaccumulating regions H produce parallel deep valleys coinciding with thestripes. Exposed parts between neighboring masks become low defectdensity single crystal regions Z or C-plane growth regions Y. Zs and Ysproduce parallel hills. The stripemask produces a crystal structure withrepeating sets of parallel hills and valleys. C-plane growth regions Ysometimes do not happen. When no C-plane growth regions Y appear, thecrystal structure has sharp hills. When C-plane growth regions Y happen,the crystal structure has blunt hills with flat tops.

Similar regions to H, Y and Z occur in the case of the dotmask case,which consists of regularly distributing isolated mask dots. Isolatedpits consisting of facets F are formed around the isolated mask dots M.The bottom of the pit coincides with the mask dots M. Portions beneaththe facets F on exposed parts become low defect density single crystalregions Z. The other portions under the C-plane on exposed parts becomeC-plane growth regions Y. Both the low defect density single crystalregions Z and the C-plane growth regions Y are low defect density singlecrystals having the same orientation.

The parts above the dots M become defect accumulating regions H in boththe stripemask case and dotmask case. The defect accumulating region isa polycrystal P, a c-axis slanting single crystal A or a c-axisinversion single crystal J. Sometimes no defect accumulating region H isyielded on a mask (O). The portion above a mask has four alternatives O,A, P and J. Namely H=O, A, P or J.

When c-axis inversion regions (=orientation inversion) J are produced(H=J) on masks, the defect accumulating regions H have Ga-planes andN-planes inverse to the other parts (Z, Y). The c-axes of the defectaccumulating regions H are inverse to the c-axes of the other parts (Z,Y). The inventors of the present invention call the orientationinversion single crystal region as a polarity inversion region. Thus thepolarity inversion region, the c-axis inversion region and theorientation inversion region are synonyms.

The interfaces between H and Z are grain boundaries K. Milder facets F′and F′ appear on the defect accumulating region H in the case of H=J.The GaN crystal grown on a dotmasked undersubstrate has plenty ofisolated pits corresponding to the mask dots on a C-plane surface. TheGaN crystal grown on a stripemasked undersubstrate has a plurality ofparallel hills and valleys corresponding to the mask stripes. In thedotmask case, the defect accumulating regions H are isolated closedregions consisting of facets F. The facets F are mainly {11-22} planesand {1-101} planes. In the stripemask case, the defect accumulatingregions H are parallel extending regions. The facets are determined bythe direction of the stripemask. Masks M are seeds of the defectaccumulating regions H.

Formation of seeds (masks) on an undersubstrate determines the positionson which defect accumulating regions H will grow. Through the decisionof positions of Hs, the positions of both low defect density singlecrystal regions Z and C-plane growth regions Y are determined by theseeds (masks). The convenient property of the mask facet growth solvesthe aforementioned problem (2) that the positions of the defectaccumulating regions H are not predetermined.

The orientation (polarity) inversion defect accumulating region Hproduces a clear, definite grain boundary K. The grain boundary Karrests dislocations firmly, prohibiting dislocations from passingthrough. The polarity inversion defect accumulating region H suppressesthe hazy release of dislocations. The positions of making defectaccumulating regions H become controllable by forming masks on anundersubstrate as seeds of Hs.

Mask positions can definitely control the positions of defectaccumulating regions H, low defect density single crystal regions Z andC-plane growth regions Y. It turns out that a defect accumulating regionH cannot always make a grain boundary K. Defect accumulating regions Hoccur on masks. But the defect accumulating region H is not always ac-axis 180 degree inversion, i.e. an orientation (polarity) inversionregion J. Sometimes an on-mask defect accumulating region H becomes apolycrystal P. At other times.another on-mask defect accumulating regionH becomes a c-axis slanting single crystal A which has an incliningc-axis different from the c-axis of the surrounding regions Z and Y.Sometimes no defect accumulating region H happens (O). On-mask regionstake four different versions O, A, P and J.

When the defect accumulating region H is a polycrystal P, some of grainshave orientation similar to the surrounding regions (Z, Y) and acontinual, clear grain boundary K does not occur around the polycrystaldefect accumulating region P. When the defect accumulating region H is ac-axis inclining single crystal A, some parts of A have orientationssimilar to the surroundings Z and Y and a continual definite boundary Kis not generated. Building a clear, continual grain boundary K requiresan on-mask 180 degree inversion c-axis region J.

When an orientation inversion region J is produced on a mask, any partof the region J has crystal lattice orientation different from thesurroundings Z and a definite powerful grain boundary is produced aroundthe orientation inversion region J. The grain boundary K has a strongpower of arresting dislocations. Without a definite grain boundary K,the defect accumulating region H has a weak power of arresting,annihilating and accommodating dislocations. Strong dislocation arrest,annihilation and accommodation ardently require the occurrence oforientation inversion defect accumulating regions J on masks. A purposeof the present invention is to provide a reliable method of making 180degree c-axis rotation polarity inversion regions J on masks as defectaccumulating regions H.

The best defect accumulating region H is an inversion region J. Apurpose of the present invention is to build polarity inversion regionsJ on masked parts for a certainty.

(1) Seeds (masks) M, which are a material capable of inhibiting GaN fromepitaxially growing thereon, are formed upon the positions on whichdefect accumulating regions H shall be grown. Here the seed mean a seedof a defect accumulating region H. The seed is a synonym for a mask.FIG. 7(1) demonstrates a section of a mask M formed on an undersubstrateU. FIG. 7(1) shows a part of the undersubstrate U having only one mask.In reality, plenty of mask stripes or dots M are formed on U. The partcovered with the mask M is called a “covered (masked) part”. The otherpart not covered with the mask is called an “exposed (unmasked) part.”

(2) Gallium nitride (GaN) is grown on the masked undersubstrate U invapor phase. Exposed parts facilitate production of crystal nuclei.Covered parts prevent nuclei from occurring. GaN crystal growth startsonly on the exposed parts. Growing crystal has a C-plane top surface onthe exposed parts. Progress of crystal growth is forbidden at the endsof the masks M. At an early stage, crystals do not overrun the sides ofthe masks M. The sides stop horizontal extension of the crystal growth.Slants starting from the sides of the mask M appear (FIG. 7(2)). Theslants are facets F which are not a C-plane. In many cases, the facetsare {11-22} planes. Stripe masks produce parallel, linear facets F whichextend in the direction vertical to the paper. Dot masks make plenty ofisolated hexagonal pits composed of facets. The stripe mask and the dotmask are similar in the growing steps following the formation of facets.Therefore the case of the stripe mask is described hereafter.

(3) Small beaks Q, which have inverse c-axes and inverse orientations,appear at middle points on the slanting facets F which have feet stoppedat the ends of masks M. The beaks Q extend in horizontal directions.FIG. 7(3) shows the beaks Q. A beak Q has an upper slant milder than thefacet F and a steep lower slant. Examination has clarified that the beakQ has orientation different at 180 degrees from the surrounding regions.Namely the beaks Q are polarity inverse regions.

(4) Progress of crystal growth increases the number of beaks Q appearingon the slanting facets F. Beaks Q dilate. Tiny beaks are unified into alarge beak on each of the facets F. The beaks Q inward expand and coverthe mask M with a gap.

(5) The beaks Q have inverse orientation. The beak has an upper facet F′whose inclination angle is smaller than the following facet F. The upperfacets F′ is one of the lower inclination angle facets {11-2-6},{11-2-5} and so forth. The lower facet of the beak Q is a facet steeperthan F′.

The beaks Q dilate in the vertical and horizontal directions. Tips ofthe paired beaks Q collide with together above the mask M. The beaks Qare unified. A bridge is formed by the coupled beaks above the mask M.Crystals having the same inverse orientation grow on the bridged beaks Qand Q. Inverse crystal buries the lower gap between the mask and thebridge. The crystal above the beaks Q has not horizontally grown fromthe facets F but has vertically grown on the inverse orientation beaksQ. The crystal above the mask has the orientation inverse to theorientation of the surrounding crystals Z and Y.

(7) The middle portion on the collision tips makes a lattice-misfittingboundary K′ therebetween and grows thick. The middle boundary K′ isdifferent from the H/Z boundary K. The inverse orientation beaks Qbecome defect accumulating regions H.

(8) When the crystal grows further (FIG. 7(5)), dislocations in the GaNcrystal are gathered onto the unified beaks above the masks M byslantingly growing facets F. Upward extension of the beaks makes defectaccumulating regions H above the masks M. A part of the gathereddislocations vanish at the grain boundaries K or the cores S of thedefect accumulating regions H. The rest are arrested and accumulated inthe grain boundaries K and the cores S. The defect accumulating regionsdeprive dislocations of other parts. The parts under the facets F becomelow dislocation single crystal regions Z.

The above processes produce defect accumulating regions H as orientationinversion regions J. Production of polarity inversion regions J on masksrequires to generate beaks Q on all the facets (for example, {11-22}facets) as shown in FIG. 7(3). Stable and allover formation of beaks Qon facets is important for inducing the polarity inversion regions J.Unless stable and allover beaks are produced, defect accumulatingregions H on masks would not become orientation inversion regions J. Inthe case, on-mask regions H cannot strongly attract dislocations fromthe surrounding regions and cannot annihilate the dislocations.Dislocations are dispersed into the neighboring regions. No low defectsingle crystal regions Z occur under the facets.

Vapor phase facet growth on a masked undersubstrate cannot necessarilymake polarity inversion regions J on masks. Formation of the beaks Q onfacets is indispensable to make orientation inversion regions J.However, it is not easy to produce stable orientation inversion beaks Qon all the facets slantingly expanding from the sides of masks.

This invention decreases dislocations by facet growth. This method canbe called a “facet growth method.” The facet growth method is entirelydifferent from the well-known epitaxial lateral overgrowth (ELO) methodwhich relies upon masks for reducing dislocations. The facet growthmethod completely differs from the ELO. But the facet growth method hasa possibility of being confused with the ELO, because both the facetgrowth method and the ELO make use of masks for decreasing dislocations.Several different points are explained for avoiding the confusion of thefacet growth method with the ELO.

(a) The ELO employs a wide mask. The ELO prepares wider masked parts andnarrower exposed parts. Masked>Exposed areas in ELO. The ELO makes amask having small windows. On the contrary the facet growth method basedon the present invention employs narrow masks. The facet growth methodprepares narrower masked parts and wider exposed parts. Masked<Exposedareas in facet growth. The facet growth method forms tiny, slim masks onan undersubstrate.

(b) The facet growth method is different from the ELO in the existenceof polarity inversion region J. The ELO makes crystals on exposed partsand allows the crystals to ride upon the mask with the same orientation.The ELO is immune from the polarity inversion GaN crystals. For example,if an ELO crystal has {11-22} facets on the verges of masks, the ELOcrystal steps on the mask and grows on the mask with maintaining {11-22}facets.

Polarity inversion does not take place in the ELO. The ELO-grown GaN isa single crystal as a whole. On the contrary, the facet growth of thepresent invention does not allow GaN crystals to step on masks butinduces formation of beaks, which have inversion polarity, on facets.GaN grows on the beaks as seeds. Thus polarity inversion regions areborn in the facet growth method. Discontinual interfaces are produced bythe polarity inversion regions.

(c) The dislocation reduction growth direction is horizontal directionsin the ELO. The ELO decreases dislocations by horizontally growing thecrystal on the mask. The dislocation reduction growth direction is thevertical direction in the facet growth. Thick growth in the verticaldirection gathers, arrests, annihilates and accommodates dislocationsinto defect accumulating regions H in the facet growth. The ELO and thefacet growth are different in the defect-reducing growth directions.

(d) The ELO makes low defect density regions on masks. On-mask regionsare low defect density crystals with high quality in the ELO. On exposedpart, crystals with high defect density are grown in the ELO. On thecontrary, the facet growth method based on the present inventionproduces low defect density GaN crystals with good quality on exposedparts. Masked parts produce low quality GaN crystals with highdislocation density in the facet growth. The ELO is entirely inverse tothe facet growth at the point which of the masked parts and the exposedparts produces low dislocation density regions or high dislocationdensity regions.

SUMMARY OF THE INVENTION

The present invention forms polarity inversion regions J on masks bypreparing an undersubstrate, forming masks having a function ofpreventing GaN from epitaxial growing partially on the undersubstrate,preparing an undersubstrate with masked parts and exposed parts,supplying a Ga-material , an N-material and a carbon-material andgrowing GaN crystals on the masked undersubstrate in vapor phase. Thegist of the present invention is that on-mask defect accumulatingregions H are surely allotted to orientation (polarity) inversionregions J by carbon doping. Conventional vapor phase growth of GaNconsists of two steps of (buffer layer formation)+(epitaxial growth).The present invention adds a step of orientation inversion regionformation between the buffer layer formation and the epitaxial growth.The vapor phase GaN growth of the present invention consists of threesteps of (buffer layer formation)+(carbon doping orientation inversionregion formation) +(epitaxial growth).

Preferable candidates of undersubstrates are a sapphire (α-Al₂O₃) (0001)single crystal wafer, a silicon (Si) (111) single crystal wafer, asilicon carbide (SiC) (0001) single crystal wafer, a GaN single crystalwafer, a GaAs (111) single crystal wafer and so on. A GaN/sapphirecomplex wafer consisting a sapphire wafer and a thin GaN layer grown onthe sapphire is another candidate for an undersubstrate. TheGaN/sapphire wafer is called a “template.”

As shown in FIG. 7(1), a mask M, which is an assemble of masking stripes(M1) or masking dots (M2), is formed upon an undersubstrate U. Theformer (M1) is named a “stripemask” in short. The latter (M2) is named a“dotmask”. The materials of the mask are silicon dioxide (SiO₂),platinum (Pt), tungsten (W), silicon nitride (Si₃N₄) and so on. Thethickness of the mask M is about 30 nm to 300 nm. The mask pattern isarbitrarily chosen. A dot mask (M2) pattern consists of plenty ofisolated masking dots aligning regularly lengthwise and crosswise. Astripe mask (M1) pattern consists of plenty of linear masking stripesaligning at a constant pitch in parallel. The present invention allowsboth the dot mask (M2) and the stripe mask (M1). Formation of a maskdivides the surface of the undersubstrate U into covered (masked) partsand exposed (unmasked) parts. The covered parts are narrow. The exposedparts are wide. This is a restriction deriving from the facet growthmethod.

Gallium nitride is grown at a low temperature on the undersubstrate. Thelow temperature grown GaN is a thin buffer layer of a thickness of about30 nm to 200 nm. The temperature for growing the buffer layer is denotedby Tb. Tb=400° C. to 600° C. The buffer layer has a function ofalleviating strain acting between the undersubstrate and the GaN layer.

The buffer layer is formed only on the exposed parts. The covered partsare still left uncovered with the buffer layer.

The gist of the present invention is carbon doping growth for promotingthe formation of c-axis inversion regions J on the mask covered parts.GaN crystal is grown on the buffer/undersubstrate with supplying a Gamaterial, a nitrogen material and a carbon material. The covered partsare still uncovered with GaN. GaN crystals grow only on the exposedparts. The parts contacting with the mask become facets (F), as shown inFIG. 7(2).

The parts following the facet (F) is named low defect single crystalregions Z. Other parts lying far from the mask become C-plane growthregions Y, which have smooth C-plane surfaces.

Carbon doping generates inward extending beaks Q at middle points onslanting facets F rising from an end of the mask M (FIG. 7(3)). Thebeaks Q have an orientation with the c-axis rotating at 180 degrees tothe surrounding regions. Namely a beak Q is a single crystal withinverse orientation. The beaks Q extend inward from the slanting facetsF. Counterpart beaks Q come to contact above the mask M and jointogether, as exhibited in FIG. 7(4). GaN crystals further grow on thejoint beaks Q as seeds. The crystals above the mask which grow on thebeaks have inverse orientation with the c-axis reversed at 180 degreesto the c-axis of the surroundings. The GaN crystals grown on the beaks Qabove the masks M are named “(c-axis) inversion regions J”. Theinversion regions J further grow upward on the covered parts withmaintaining constant sections which are slightly narrower than thecovered parts or masks M (FIG. 7(5)). The inversion regions J attractdislocations from the surrounding regions and accumulate dislocationswithin the inversion regions J.

The HVPE method employs a Ga melt as a Ga material and supplies an HClgas. HCl gas reacts with the Ga melt and synthesizes gallium chloride(GaCI). GaCl reacts with ammonia (NH₃) gas and makes gallium nitride(GaN). GaN is plied on the undersubstrate. The present invention aims toconfirm the formation of inversion regions on masks by doping GaN withcarbon (C). This invention uses hydrocarbon gases or solid carbon asmaterials of carbon. The HVPE growth is done under the atmosphericpressure (1 atm=0.1 MPa). When a hydrocarbon gas is used as a carbonmaterial, the present invention should supply the hydrocarbon gas of apartial pressure between 1×10⁻⁴ atm (10Pa) and 5×10−² atm (5 kPa). Foran early stage growth of forming inversion regions (H), the growingtemperature should be 900° C. to 1100° C. Preferable range of thegrowing temperature is 990 ° C. to 1050° C. The growing speed is 50 μm/hto 100 μm/h. The time for making the orientation inversion regions is0.5 hour to 2 hours.

Maintaining facets gathers dislocations on masks. On-mask regions inwhich many dislocations are accumulated are called defect accumulatingregions H. The defect accumulating regions H take three differentcrystal structures. First alternative of H is an axis-inclining singlecrystal A having a slantingly upward c-axis. Second alternative of H isa polycrystal P. Third alternative is a c-axis inversion region J whichhas the c-axis reversed at 180 degrees to the c-axis of other regions Yand Z. Sometimes defect accumulating regions H are not generated onmasks (case O). H has four alternatives O, P, A and J (H=0, P, A or J).

The present invention aims at making c-axis inversion regions J on masksas defect accumulating regions H. H=J is a purpose of the presentinvention. The on-mask regions H have a function of extractingdislocations existing on the surrounding portions under the facets andarresting the dislocations as defect accumulating regions. Sincedislocations are eliminated from the surrounding portions, thesurrounding portions become low defect density single crystal regions Z.The function of extracting and arresting dislocations is stronger inorder of the polarity inversion J, the polycrystal P, c-axis slantingsingle crystal A and non-occurrence 0. The following inequalityintuitively denotes the order of the power of O, A, P and J.non-occurrence O<c-axis slanting single crystal A<polycrystal P<polarityinversion J.

The polarity inversion J is the best for the defect accumulating regionH. J is the best H. The present invention has searched the condition ofassigning polarity (orientation) inversion regions J to the defectaccumulating regions H and succeeded in making polarity inversionregions J on masks without fail.

The cathode luminescence (CL) can identify which is an on-mask defectaccumulating region of A, P and J. A, P and J are discernible for the CLobservation. Fluorescence microscope observation is available fordiscerning A, P and J. Human eye sight is incompetent, because GaNcrystal is uniformly transparent.

Then thick GaN crystals are grown epitaxially on the maskedundersubstrates with inversion regions J formed on the mask for a longtime. The time of thick GaN crystal growth, which is contingent on thethickness of object GaN crystals, is several tens of hours, severalhundreds of hours or several thousands of hours. The temperature of athick GaN crystal is denoted by the second growth temperature Te fordiscerning from Tj which is the temperature of making the polarityinversion regions J. The second growth temperature has an appropriaterange between 990° C. and 1200° C., i.e. Te=990° C. to 1200° C. The morepreferable range is Te=1000° C. to 1200° C.

ADVANTAGES OF THE INVENTION

Low defect density gallium nitride substrates with high quality havebeen required. The facet-growth method is promising, because the facetgrowth method can reduce dislocations by forming masks (dots or stripes)on an undersubstrate, growing GaN crystals in vapor phase, producingfacets (pits or grooves) originating from the masks, making defectaccumulating regions H on the masks, maintaining the facets and sweepingdislocations from the surrounding regions into the defect accumulatingregions H. The defect accumulating region H has several candidates. Theorientation inversion region J is the optimum for the defectaccumulating region H. The inventors have found out that stableformation of the orientation inversion regions J having an inversec-axis on the masks depends upon the crystal growth condition at anearly stage of growth.

If the early stage growth condition is inappropriate, the defectaccumulating regions H upon masks become polycrystal P or c-axis upwardinclining single crystals A. The polycrystal P or c-axis upwardinclining single crystals A have insufficient in attracting andarresting dislocations from neighboring crystal regions. It is stronglydesired to establish the defect accumulating regions H on the masks asorientation inversion regions J.

The steps (3), (4), (5) and (6) mentioned above are early stages offorming the orientation inversion regions J (beaks Q). Generation of thebeaks Q is so important. What is the essential condition to generate thebeaks Q? This invention has found out that carbon doping at an earlystage of growth induces the occurrence of beaks Q and orientationinversion regions J following the beaks Q.

The early stage growth time with carbon doping for preparing the beaks Qand inversion regions J is about 0.5 hour to 2 hours. At the end of theearly growth stage with carbon doping, defect accumulating regions Hwhich are orientation inversion regions J have made on masked parts, andlow defect density single crystal regions Z have been made on exposedparts. Sometimes C-plane growth regions Y have been made at middles onexposed parts. Sometimes no C-plane growth regions Y are produced.

The present invention confirms the formation of the orientationinversion regions J by doping the growing GaN crystal with carbon. Theformation of the orientation inversion regions J on the masked partsenables the defect accumulating regions H to attract, gather and arrestdislocations from the neighboring single crystal regions on the exposedparts. The neighboring regions become low defect density single crystalregions Z. Thus low defect density GaN crystals with high quality areobtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An HVPE method, MOCVD method, MOC. method and sublimation method areable to grow GaN crystals. The present invention employs the HVPE methodfor making orientation inversion regions on masks by carbon doping.MOCVD and MOC. methods use carbon-including Ga materials (metallorganicgases, e.g., trimethylgallium TMG, trimethylgallium TEG). It is stillunclear whether the carbon included in the metallorganic gases in MOCVDand MOC. methods produces orientation inversion regions or not.

The present invention aims at GaN crystals grown by the HVPE (hydrideVapor Phase Epitaxy) method. The HVPE method uses a tall hot-wallfurnace. The hot-wall HVPE furnace has a circular heater which isdivided into several heater elements in the vertical direction forproducing an arbitrary temperature distribution in the verticaldirection. The furnace has a Ga-boat maintaining Ga metal in an upperspace and a susceptor for supporting a specimen in a lower space.

The present invention grows GaN crystals in an HVPE furnace kept at theatmospheric pressure (1 atm=100 kPa). The furnace heats the Ga-boat at atemperature higher than 800° C. for melting Ga into a melt. The Ga-boatcontains a Ga melt. Gas inlet pipes are furnished at the top of thefurnace. A mixture of hydrogen and hydrochloride gases (H₂+HCl) is blownvia a gas inlet pipe to the Ga melt in the Ga-boat. Reaction of HCl withGa synthesizes gallium chloride (GaCl). GaCl is gaseous. GaCI gas fallsand comes close to the heated susceptor and specimen. Another mixture ofhydrogen and ammonia gases (H₂+NH₃) is blown via another gas inlet pipein the vicinity of the heated susceptor. GaCl reacts with NH₃. GaN issynthesized. Synthesized GaN piles on the specimen.

The mask patterns formed on an undersubstrate are made of a materialcapable of preventing GaN from epitaxially growing thereon. Favorablemask materials are silicon dioxide (SiO₂), silicon nitride (SiN),platinum (Pt) and tungsten (W). Masks become seeds of defectaccumulating regions H. An undersubstrate determines the orientation ofGaN growing thereon. Directions of the masks determine the directions offacets aligning to the mask sides. Masks having sides satisfying acertain relation to the orientation of the undersubstrate should beprepared.

Embodiment 1 Dependence upon first growth temperature Te

[1. Undersubstrate (U)]

Three kinds U1, U2 and U3 of undersubstrates are prepared. U1 is 2-inchdiameter sapphire (Al₂O₃) single crystal substrates. U2 is 2-inchdiameter gallium arsenide (GaAs) single crystal substrates. U3 is 2-inchdiameter sapphire substrates covered with a 1.5 μm thick GaN epitaxiallygrown by an MOCVD method. The sapphire undersubstrates (U1) are C-plane(0001) surface wafers. The GaAs undersubstrates (U2) are (111)A-planewafers. The GaN/sapphire undersubstrates (U3) have a mirror (0001) GaNsurface. A GaN/sapphire wafer is sometimes called a “template”.

[2. Mask patterns (M)]

0.1 μm thick SiO₂ films are produced on the three kinds ofundersubstrates U1, U2 and U3. Two kinds of patterns are formed byphotolithography and etching. One is a stripe pattern (M1) havingparallel mask stripes. The other is a dot pattern (M2) having isolatedmask dots. The parts which are not covered with masks are named exposedparts. The parts which are covered with masks are named covered (masked)parts. GaN begins to grow on exposed parts.

(M1: Stripe Type Mask Pattern; FIG. 8)

FIG. 8(1) shows a stripe type mask pattern M1 having parallel linearmask stripes formed on an undersubstrate U. The stripes align inparallel at a common pitch p. The width of a mask stripe is s. The widthof an exposed part is (p-s). GaN starts growth on exposed parts.

When GaN is grown on a masked undersubstrate by the facet growth method,linear defect accumulating regions (H) are produced upon the masks M asshown in FIG. 8(2). Low defect density single crystal regions Z areproduced on exposed parts adjacent to the masks M. C-plane growthregions Y are sometimes produced at middles of the exposed parts and atother times no C-plane growth region Y appears.

A direction of extending stripes is determined to be parallel with<1-100>direction of the GaN crystal growing on the maskedundersubstrate. The mask preparation precedes GaN growth. However, thereis a definite relation between the undersubstrate orientation and GaNorientation grown on the undersubstrate. The directions of growing GaNcrystal are deducible from the undersubstrate directions. A GaN crystalgrown on an C-plane sapphire undersubstrate U1 has a 90 degree rotatingorientation along the c-axis from the orientation of the sapphireC-plane undersubstrate U1. A GaN crystal grown on a GaAs(111)undersubstrate U2 has a similar orientation to the on-sapphire GaN withregard to three forehand numbers of Miller index. A GaN crystal grown ona GaN/sapphire undersubstrate (template) U3 has the same orientation asthe GaN film. Determination of the extending direction of stripes withregard to the orientation of the undersubstrate succeeds in coincidingthe mask extending direction with <1-100> direction of GaN crystal grownon the undersubstrate.

In the case of the GaN/sapphire undersubstrate (U3), the mask stripesshould be determined in parallel to <1-100> direction of GaN. In thecase of the (111) GaAs undersubstrate (U2), the mask stripes should bedetermined in parallel to <11-2> direction of GaAs. In the case of thesapphire undersubstrate (U1), the mask stripes should be determined inparallel to <11-20> direction of sapphire.

The stripemask pattern has parallel stripes with a width s of 30 μm,i.e. s=30 μm which are repeated at a pitch p of 300 μm, i.e. p=300 μm.Parallel exposed parts extend in the same direction. An exposed part hasa width e of 270 μm, i.e. e=270 m. The pitch p is the sum of a maskedpart width s and an exposed part width e. p=e+s. The pitch is a distancefrom the middle of an exposed part to the middle of a neighboring exposepart. The exposed:masked part ratio (e/s) is 9:1.

(M2: Dotmask pattern; FIG. 10)

A dotmask pattern M2 is composed of aligning dot trains with adiscrepancy of half a pitch as shown in FIG. 10 (1). The patternincludes small round dots M placed at the corners of similar regulartriangles which occupy the surface of an undersubstrate U without gap.Masked parts are narrower than exposed parts. Most of the parts areexposed parts.

Crystals start to grow on the exposed parts.

FIG. 10(2) shows a structure of a GaN crystal prepared by growing a GaNcrystal on the dotmasked undersubstrate of FIG. 10(1) and eliminatingthe undersubstrate from the GaN crystal. Defect accumulating regions Hare produced on mask dots M. Low defect density single crystal regions Zappear on exposed parts around the defect accumulating regions H.

A continual C-plane growth region Y occurs on an extra exposed partwhich is not covered with the low defect density single crystal regionsZ.

In the case of a dotmask pattern M2, mask dots are arranged in symmetricspots.

The dot mask pattern is a set of dots lying on corner points of aplurality of regular triangles aligning two-dimensionally without gap.For example, a dotmask pattern with six fold rotation symmetry isemployed. The direction of the trains of dots M is determined inparallel to GaN<1-100> direction. There is a definite relation betweenthe undersubstrate orientation and the GaN orientation. Although GaNcrystal is grown on an undersubstrate after forming the mask dots, theGaN<1-100> direction can be predetermined. In the case of a sapphireundersubstrate (U1), dot trains should be parallel to the sapphire<11-20> direction, since GaN<1-100>is generated to be parallel to sapphire<11-20>. In the case of a GaAs(111) undersubstrate (U2), dot trainsshould be parallel to the GaAs<11-2> direction.

A mask dot is circular. The diameter t of a dot is t=50 μm. The pitch pof a dotmask is p=300 μm. The pitch p is a distance between the nearestdot centers. The length f of an exposed part from a dot to a closest dotis f=250 μm. The unit regular triangle having three nearest dots has anarea of 38971 μm². The dot has an area of 1963 m . 38971/1963=19.8.

The exposed:masked area ratio is about 1.9:1.

[3. Growth of forming buffer layers and orientation inversion regions J]

Embodiment 1 places M1, M2-masked undersubstrates U1, U2 and U3 (M1U1,M1U2, M1U3, M2U1, M2U2 and M2U3) on a susceptor in an HVPE furnace. Atthe initial stage, GaN buffer layers are grown for 15 minutes at a lowtemperature of about Tb=500° C. under an ammonia partial pressureP_(NH3)=0.2 atm (20 kPa) and a hydrochloride partial pressureP_(HCl)=2×10⁻³ atm (0.2 kPa). The thickness of the GaN buffer layers is60 nm.

Then the specimens are heated up to an inversion region formationtemperature TJ=1000° C. At Tj, orientation inversion regions J are grownon the masks and epitaxial crystals are grown on exposed parts for aboutone hour. Material gases are an (H₂+HCl) gas, an (H₂+NH₃) gas and ahydrocarbon gas. The hydrocarbon gas is either a methane gas or anethane gas. The ammonia partial pressure is P_(NH3)=0.2 atm (20 kPa).The hydrochloride partial pressure is P_(HCl)=2×10⁻² atm (2 kPa). Somespecimens are grown without supplying a hydrocarbon gas as comparisonexamples. After one hour growth, the specimens are cooled and are takenout of the furnace without further thick crystal growth. The optimumrange of forming the orientation inversion regions J is Tj=970° C. to1100° C. Appropriate time for inducing the orientation (polarity,c-axis) inversion regions J is 0.5 hour to 2 hours at an early stage ofgrowth.

[4. Kinds of hydrocarbon gases and hydrocarbon partial pressure P_(HC)]

The present invention dopes a growing GaN crystal with carbon bysupplying solid carbon or a hydrocarbon gas into a reaction furnace forpreparing the orientation inversion regions J on masked parts. Methane(CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂) and otherhydrocarbon gases are available for forming the inversion regions J.

Appropriate range of the hydrocarbon gas partial pressure isP_(HC)=1×10⁻⁴ atm (10 Pa) to P_(HC)=5×10⁻² atm (5 kPa). The followingexperiments adopt three kinds of hydrocarbon partial pressures.

-   (1) Methane gas (CH₄) P_(HC)8×10⁻³ atm(0.8 kPa).-   (2) Ethane gas (C₂H₆) P_(HC)=8×10⁻³ atm(0.8 kPa).-   (3) No hydrocarbon P_(HC)=0.    [5. Crystal growth for inducing orientation inversion regions J]

Experiments have clarified the steps required to build 180 degreeinverting c-axis regions J on masks.

FIGS.7(1), (2), (3), (4) and (5) demonstrate the steps of making thec-axis inversion regions J as defect accumulating regions H. FIG. 7(1)shows the formation of a stripemask on an undersubstrate U. Thought thefigure shows only one mask, many identical masks M are formed on theundersubstrate as shown in FIG. 8(1). Instead of the stripemask, anarbitrary mask is available. The mask has a function of suppressingepitaxial growth of GaN. The masked undersubstrate is laid upon asusceptor in a reaction furnace, for example an HVPE furnace. GaN isgrown on the masked undersubstrate MU in vapor phase. Initially GaN doesnot grown on the masks M. GaN starts to grow on exposed parts as shownin FIG. 7(2). Without stepping on the masks, GaN crystals pervadeallover the exposed parts as films. Further growth increases the heightsof the GaN crystals on the exposed parts. Slants starting from sides ofthe masks to tops of the GaN crystals are produced. The slants growfurther without overstepping the masks. The slants are facets F having adefinite inclination angle. The orientation of the facets F depends uponthe direction of the masks. For example, the slants are {11-22} facets.There is no GaN crystal on the masks. A pair of facets F confront eachother over a mask M.

The inventors have noticed a sign of building the orientation inversionregions J. Preceding the formation of orientation inversion regions J,rugged protrusions Q and Q appear at middle heights on the facets F. Theprotrusions Q are named beaks Q. Since the facets F and F confront eachother, a pair of beaks Q and Q also confront each other (FIG. 7(3)). Thebeaks Q become seeds of the orientation inversion regions J. Theorientation inversion regions J follow the beaks Q. Unless the beaks Qhappen, no orientation inversion regions J occur. The beaks Q invite theorientation inversion regions J on masks. An upper surface of the beak Qinclines at an inclination angle between 25 degrees and 35 degrees tothe horizontal plane. The beaks Q are single crystals having orientation180 degrees inverting to the orientation of the neighboring facets F.Since the orientation is inverse in the beaks Q, the beaks Q arepossible to become seeds of the orientation inversion regions J. Therugged beaks Q grow and extend further. Tips of the extending beaks Qand Q come in contact with each other. Then the beaks Q and Q areunified into a bridge as shown in FIG. 7(4). The bridge has theinversion orientation. The bridges are seeds of orientation inversionregions J.

As shown in FIG. 7(4), the space above the mask is covered with theunified beaks Q. The beaks Q have no contact with the masks. The beaksexpand from intermediate heights of the facets F in the horizontaldirection, meet with together and make a bridge. The both side facets Fand F are bridged by the unified beaks Q. After the unification,crystals having the same orientation as the beaks Q grow on the unifiedbeaks Q in the vertical direction. The beaks have c-axis inversion(orientation inversion) crystals. The crystals growing on the beaks Qbecome orientation inversion crystals J. As shown in FIG. 7(5), crystalswith the same orientation as the beaks Q vertically grow on the beaks Q.The crystals which grow on the masked parts are defect accumulatingregions H. The defect accumulating regions H become orientationinversion regions J. Higher crystals grow on both exposed parts. Closerinclining surfaces of the higher crystals are the facets F, which isshown by FIG. 7(4).

The on-exposed part crystals have plenty of dislocations generated atthe interfaces between the undersubstrate and the growing crystals.Dislocations extend in the same direction as the growth direction. Thefacet growth method continues the growth without burying the facet pitsor facet grooves.

Crystals on the facets grow in the direction vertical to the facets.Dislocations extend in the outward slanting direction which is inparallel with a normal standing on the facet. Dislocations extend towardthe defect accumulating regions H on the masks. The dislocations attainat the defect accumulating regions H. The dislocations are arrested inthe defect accumulating regions H. The once arrested dislocations neverreturn to the facets F. Dislocation density in the crystals just belowthe facets F decreases. The crystals grown on the exposed parts belowthe facets are named low defect density single crystal regions Z.

Initially the regions have plenty of dislocations generated between theundersubstrate and the growing crystals. The facet growth extracts thedislocations out of the regions and transfers the dislocations into thedefect accumulating regions H on the masks. The regions become lowdefect density. The orientation of the regions is determined to therelation with the undersubstrate. Thus the regions become low defectdensity single crystal regions Z. The interfaces between the low defectdensity single crystal regions Z and the defect accumulating regions Hare grain boundaries K and K. The orientation is abruptly reversed atthe grain boundaries K. The dislocations which have been once arrestedin the defect accumulating regions H are not released again. The defectdensity of the neighboring regions Z becomes lower and lower with theprogress of the facet growth.

The facet growth has been maintained until the end of the growth. Longtime exclusion of dislocations from the exposed parts further enhancesthe quality of the low defect density single crystal regions Z due tothe decrement of dislocations.

On the contrary sometimes the facet growth cannot produce inverseorientation regions J on masks. This invention has discovered thatcarbon doping is effective in producing the orientation inversionregions J on masks in the facet growth. The present invention makesorientation inversion regions J on masked parts at an early stage ofgrowth by carbon doping, grows a thick GaN crystal film for a long timewithout carbon doping and makes low defect density GaN crystals of highquality. The main purpose of the present invention is production oforientation inversion regions J. Without further producing a thick GaNfilm, samples are cooled and are taken out of the furnace after theearly stage of growth for examining whether the orientation inversionregions J appear on masked parts. All the samples have thicknesses ofabout 70 μm. The growth speed is about 70 μm/h.

[6. Observation of occurrence or non-occurrence of orientation inversionregions on masks]

-   [(1) In the case of Methane gas (CH₄); P_(HC)=8×10⁻³ atm (800 Pa)]-   Undersubstrates=sapphire substrate (U1), GaAs substrate (U2),    GaN/sapphire substrate(U3).-   Mask pattern=stripe mask (M1), dot mask (M2).-   Result of observation-   M1; stripe mask; intermittent orientation inversion regions J appear    like wavy lines on mask-   stripes (U1, U2 and U3).-   M2; dot mask; orientation inversion regions J occur on almost all of    the mask dots (U1, U2 and U3).-   [(2) In the case of Ethane gas (C₂H₆); P_(HC)=8×10⁻³ atm (800 Pa)]-   Undersubstrates=sapphire substrate (U1), GaAs substrate (U2),    GaN/sapphire substrate(U3).-   Mask pattern=stripe mask (M1), dot mask (M2).-   Result of observation-   M1; stripe mask; intermittent orientation inversion regions J appear    like wavy lines on mask stripes (U1, U2 and U3).-   M2; dot mask; orientation inversion regions J occur on almost all of    the mask dots (U1, U2 and U3).-   [(3) In the case of non-hydrocarbon gas; P_(HC)=0]-   Undersubstrates=sapphire substrate (U1), GaAs substrate (U2),    GaN/sapphire substrate(U3).-   Mask pattern=stripe mask (M1), dot mask (M2).-   Result of observation-   M1; stripe mask; few intermittent orientation inversion regions J    appear on mask stripes (U1, U2 and U3).-   M2; dot mask; orientation inversion regions J occur on few mask dots    (U1, U2 and U3).

The results teach us that orientation inversion regions J occurintermittently on masks when no hydrocarbon gas is supplied. Supply ofhydrocarbon gas promotes the occurrence of orientation inversion regionsJ on masks. Methane and ethane are equivalently suitable for a carbondoping gas. 800 Pa of hydrocarbon partial pressure develops theorientation inversion regions J on masks but is still insufficient fororientation inversion regions J to occupy all the masks. Further higherhydrocarbon (CH₄, C₂H₆, etc.) partial pressure is required oforientation inversion regions in order to prevail on all the masks.

Embodiment 2 (Solid Carbon)

Embodiment 2 grows GaN crystals on SiO₂ masked (M1 and M2) C-planesapphire undersubstrates (U1), GaAs undersubstrates (U2) andGaN/sapphire undersubstrates in the same furnace as Embodiment 1 for 60minutes with a supply of carbon. Embodiment 2 differs from Embodiment 1in the method of carbon supply. Instead of supplying hydrocarbon gases,Embodiment 2 uses solid carbon. A carbon plate is placed at a highertemperature part set at an upstream of the growth part (susceptor) inthe HVPE furnace. The other conditions are similar to Embodiment 1.

Undersubstrates (U1, U2 and U3) with stripe and dot masks (M1; M2) areput on the susceptor in the furnace.

At an early stage, GaN buffer layers are grown for 15 minutes on the M1,M2-masked undersubstrates (U1, U2 and U3) at a low temperature of about500° C. (Tb=500° C.) under an ammonia partial pressure P_(NH3)=0.2 atm(20 kPa) and a P_(HCl) partial pressure P_(HCl)=2×10⁻³ atm (0.2 kPa).The thickness of the GaN buffer layers is about 60 nm.

The temperature is raised up to an inversion region generatingtemperature Tj=1000° C. Orientation inversion regions and epitaxialgrowth regions are grown on masked parts and exposed parts respectivelyfor about one hour at 1000° C. under P_(NH3)0.2 atm (20 kPa) andP_(HCl)=2×10⁻² atm (2 kPa). Carbon is supplied from the carbon plateplaced between the Ga-boat and the susceptor. After one hour growth,samples are cooled and taken out of the furnace without further growthfor examining occurrence or non-occurrence of orientation inversionregions.

-   Results of observation-   Undersubstrates; sapphire (U1), GaAs (U2), GaN/sapphire (U3)-   GaN film thicknesses: 70 μm-   M1(stripe mask): intermittent orientation inversion regions J occur    like wave lines on mask stripe.-   M2(dot mask): Orientation inversion regions J appear on almost all    the mask dots.

It is confirmed that Embodiment 2 which has a carbon plate as a carbonsource can produce orientation inversion regions J on masked parts.Similar results are observed on the GaN films grown on the sapphire(U1), GaAs (U2) and GaN/sapphire undersubstrates. There is no differencein the kinds of undersubstrates. The GaN is transparent without beingcolored black or yellow.

After the growth, the carbon plate placed in the furnace is taken out.The weight of the carbon plate is measured. The weight of the carbonplate has been reduced. The carrier gas is hydrogen (H₂). An equivalentpartial pressure of hydrocarbon is calculated on the assumption that allthe loss of weight of carbon would be converted into methane gas CH₄.The methane partial pressure is calculated to be P_(HC)=1×10⁻² atm (1kPa) by taking account of the gas flow velocity. P_(HC)=1 kPa is safelyincluded within the scope of 10 Pa to 5 kPa.

Embodiment 2 teaches us that solid carbon has the same effect in makingorientation inversion regions as hydrocarbon gases. Instead of supplyinggaseous hydrocarbons, solid carbon is also available.

When a carbon plate is placed in the furnace, the tens hour to thousandshour growing thick GaN crystals following the 0.5 hour to 2 hour longformation of the orientation inversion regions J are also doped withcarbon. When carbon containing GaN crystals are undesirable, the solidcarbon method is inappropriate.

Embodiment 3 (Relation Between Hydrocarbon Partial Pressure P_(HC) andFormation of Orientation Inversion Regions J

Embodiment 3 examines the dependence of formation of orientationinversion regions J upon hydrocarbon partial pressure by varying thepartial pressure (flow rate) of gaseous carbon material similar toEmbodiment 1. Embodiment 3 uses the same HVPE furnace as Embodiment 1.Embodiment 3 employs GaAs(111) A-plane wafers (U2) as undersubstrates. Adot-mask (M2) is formed on a GaAs(111) undersubstrate (U2). This is adotmasked GaAs undersubstrate (M2U2). A stripe mask (M1) is formed onanother GaAs(111) undersubstrate (U2). This is a stripemasked GaAsundersubstrate (M1U2).

Buffer layers and orientation inversion regions J are produced upon twokinds of undersubstrates (M1 U2, M2U2). Change of the inversion regionformation is examined by varying the supply of hydrocarbon gases.

Embodiment 3 places the above specimens on a susceptor in an HVPEfurnace.

GaN buffer layers are grown for 15 minutes at a low temperature of about500° C.(Tb) under a NH₃ partial pressure of P_(NH3)=0.2 atm (20 kPa) andan HCl partial pressure of P_(HCl)=2×10⁻³ atm (0.2 kPa). Theammonia/hydrochloride ratio is P_(NH3)/P_(HCl)=100. The thickness of theGaN buffer layer is about 60 nm.

The specimens are further heated up to a temperature of Tj=1000° C. forinducing orientation inversion regions J by carbon doping, i.e. bysupplying methane gas. The NH₃ partial pressure is P_(NH3)=0.2 atm (20kPa). The HCl partial pressure is P_(HCl)=2×10⁻² atm (2 kPa). Theammonia/hydrochloride ratio is P_(NH3)/P_(HCl)=10.

There is a definite relation between the partial pressure of a gas andthe flow of the gas. Mass flow controllers, etc. control the flows ofgases. Embodiment 3 maintains the whole pressure of the HVPE furnace at1 atm (0.1 MPa: the atmospheric pressure). The sum of the gas flows(total flow) is known. The partial pressures of each gas can becalculated from the flow of the gas by dividing the gas flow by thetotal flow and multiplying the quotient by the total pressure. Thepartial pressures of NH₃, HCl and CH₄ are values calculated from themeasured flows of NH₃, HCl and CH₄, respectively

Embodiment 3 gives 60 minutes to the HVPE furnace for formingorientation inversion regions J on growing specimens. The thickness ofthe grown GaN layer is about 70 μm. The growing speed is about 70 μm/h.Embodiment 3 assigns seven different values to the methane partialpressure P_(CH4).

-   (1) P_(CH4)1=5×10⁵ atm (5 Pa)-   (2) P_(CH4)2=1×10⁻⁴ atm (10 Pa)-   (3) P_(CH4)3=1×10⁻³ atm (10 kPa)-   (4) P_(CH4)4=5×10⁻³ atm (500 Pa)-   (5) P_(CH4)5=1×10⁻² atm (1 kPa)-   (6) P_(CH4)6=5×10⁻² atm (5 kPa)-   (7) P_(CH4)7=1×10⁻¹ atm (10 kPa)

The specimens are cooled and taken out of the furnace. The specimens areobserved by a substantial microscope and a scanning electron microscope(SEM) for examining how the occurrence of the orientation inversionregions J depends upon the methane partial pressure P_(CH4).

-   (1) In the case of P_(CH4)1=5×10⁻⁵ atm (5 Pa)-   Result of observation:-   M1: Stripe mask: Intermittent inversion regions appear on the mask    stripes.-   M2: Dot mask: Intermittent inversion regions are generated upon few    of the mask dots.-   (2) In the case of P_(CH4)2=1×10⁻⁴ atm (10 Pa)-   Result of observation:-   M1: Stripe mask: Intermittent wavy inversion regions occur on the    mask stripes.-   M2: Dot mask: Inversion regions are generated upon most of the mask    dots.-   (3) In the case of P_(CH4)3=1×10³ atm (100 Pa)-   Result of observation:-   M1: Stripe mask: Continual inversion regions lie on all the mask    stripes.-   M2: Dot mask: Inversion regions are generated upon all of the mask    dots.-   (4) In the case of P_(CH4)4=5×10⁻³ atm (500 Pa)-   Result of observation:-   M1: Stripe mask: Continual inversion regions lie on the mask    stripes.-   M2: Dot mask: Inversion regions are generated upon all the mask    dots.-   (5) In the case of P_(CH4)5=1×10⁻² atm (1 kPa)-   Result of observation-   M1: Stripe mask: Continual inversion regions lie on the mask    stripes.-   M2: Dot mask: Inversion regions are generated upon all the mask    dots.-   (6) In the case of P_(CH4)6=5×10−2 atm (5 kPa)-   Result of observation-   M1: Stripe mask: Crystal is colored black. Wavy inversion regions    intermittently lie on the mask stripes.-   M2: Dot mask: Crystal is colored black. Inversion regions are    generated upon some of the mask dots.-   (7) In the case of P_(CH4)7=1×10⁻¹ atm (10 kPa)-   Result of observation-   M1: Stripe mask: Crystal is turned black. Cracks occur on the whole    surface.-   M2: Dot mask: Crystal is turned black. Cracks occur on the whole    surface.

P_(CH4)1=5 Pa, which makes intermittent inversion regions on masks, isinappropriate. P_(CH4)7=10 kPa, which colors the whole crystal black, isinappropriate. Appropriate range of methane partial pressures for theinversion region formation on masks is from P_(CH4)2=1×10⁴ atm (10 Pa)to P_(CH4)6=5×10⁻² atm (5 kPa).

The scope of P_(CH4) capable of inducing inversion regions on all masksand avoiding black colored crystals is P_(CH4)3=1×10⁻³ atm (100 Pa) toP_(CH4)5=1×10⁻² atm (1 kPa). Namely the formation of inversion regionsrequires a methane partial pressure P_(CH4) in a range between 10 Pa and5 kPa. P_(CH4) has a more preferable range between 100 Pa and 1 kPa.

The above result is obtained in the case of supplying gaseoushydrocarbon for carbon doping. A similar effect is realized also in thecase of placing solid carbon in the furnace for reacting with hydrogengas to produce hydrocarbon gases at a partial pressure within the abovescope.

Embodiment 4 (inversion region formation, thick GaN growth, grinding,polishing, slicing to wafers)

Embodiment 4 examines the wafers produced by forming buffer layers,making orientation inversion regions J, growing thick GaN crystals,slicing the GaN crystals into wafers, grinding the wafers and polishingthe wafers.

Masked undersubstrates (M1U1, M2U1) are prepared by forming a stripemask (M1) and a dot mask (M2) on sapphire undersubstrates (U1).Embodiment 4 places the masked undersubstrates (M1U1,M2U1) on asusceptor in an HVPE furnace and forms buffer layers on the maskedundersubstrates at a low temperature Tb=500° C. under an NH₃ partialpressure 0.2 atm (20 kPa) and an HCl partial pressure 2×10⁻³ atm(200Pa). The growth time is 15 minutes. The thickness of the buffer layer is60 nm.

The temperature of the susceptor is raised to Tj=1000° C. Embodiment 4grows epitaxial GaN layers on the buffer/masked undersubstrates (M1U1,M2U1) at 1000° C. under an NH₃ partial pressure of 0.2 atm (20 kPa), anHCl partial pressure of 3×10² atm(3 kPa) and a CH₄ partial pressure of8×10⁻³ atm (800 Pa) for 15 hours, cools specimens consisting of grownGaN crystals and masked undersubstrates and takes the specimens out ofthe furnace.

Embodiment 4 obtains thick GaN crystals with about 15 mm thickness onthe undersubstrates (M1U1, M2U1). The growth speed is 100 μm/h. Surfacesof the GaN crystals are observed by an optical microscope and a scanningelectron microscope (SEM).

FIG. 6(1) shows a perspective view of a part of a specimen (GaN/M1U1) ofgrowing GaN crystal on the stripe-masked undersubstrate (M1U1). FIG.6(2) shows a perspective view of a part of another specimen (GaN/M2U1)of growing GaN crystal on the dot-masked undersubstrate (M2U1). Thestripe mask (M1) makes a groove-structured GaN crystal havingrepetitions of parallel hills and valleys. It is observed that Valleybottoms coincide with the mask stripes. The dot mask (M2) makes a GaNcrystal having a flat surface and many isolated pits distributing on theflat surface. It is observed that the pit bottoms coincide with the maskdots. It is confirmed that lower inclination angle facets are formed atthe bottoms of the pits and valleys. It is identified that the lowerinclination angle facets should be {11-2-6} planes having an inversec-axis. There are other steeper facets at intermediate regions in thepits or the grooves. It is assumed that the steeper facets should be{11-22} planes. This means the occurrence of orientation inversionregions J at the positions of the mask stripes or dots.

The sapphire undersubstrates (U1) are eliminated by grinding.Freestanding GaN crystals are obtained. Surfaces are ground andpolished. Smooth GaN mirror wafers are obtained. The GaN wafers aretransparent. Structures are indiscernible for human eye sight. Thesmooth surfaces of the GaN wafers are examined by an optical microscopeand cathode luminescence (CL). Parallel linear grooves of a width of 20μm regularly aligning at a pitch of 300 μm are observed in thestripemasked (M1) specimen. The grooves are defect accumulating regionsH. The grooves derive from the occurrence of {11-2-6} planes. Theexistence of {11-2-6} planes signifies that the defect accumulatingregions H are orientation inversion regions J. The GaN wafer has theHZYZHZYZ structure as shown in FIG. 8(2).

The dotmasked (M2) specimen reveals concavities with a 30 μm to 40 μmdiameter arranged at a pitch of 300 μm at spots having six fold rotationsymmetry. The concavities correspond to the positions of the mask dots.FIG. 10(2) shows a CL image of a part of the GaN crystal produced on thedotmask (M2). The crystal has a concentric structure consisting of adefect accumulating region H, a low defect density single crystal regionZ and a C-plane growth region Y. Comparison FIG. 10(2) with FIG. 10(1)confirms that the defect accumulating regions H are generated on themask dots and the low defect density single crystal regions Z and theC-plane growth region Y are produced on the exposed parts.

CL observation shows a threading dislocation appearing on the surface asa dark spot. Etch pit density (EPD), which is a density of the threadingdislocations, is measured by counting dark spots in a definite area onthe CL image. Defect accumulating regions H have a high EPD of 10⁷ cm⁻²to 10⁸ cm⁻². Low defect density single crystal regions Z and C-planegrowth regions Y have low EPDs of about 1×10⁵ cm⁻². It is confirmed thatwide low defect density single crystal regions Z with a low EPD aregenerated. The GaN crystals made in Embodiment 4 are inhomogeneous GaNsubstrates containing Zs, Y and Hs. But the positions and areas of Zs, Yand Hs are clearly predetermined in the GaN substrates. The GaNsubstrates of the present invention enable to provide low defect densityGaN substrate wafers for making blue/violet laser devices enjoying highquality.

Elements contained in the GaN crystals grown by Embodiment 4 aremeasured by SIMS (Secondary Ion Mass Spectroscopy) for examining whethercarbon atoms are absorbed in the grown GaN substrates or not.

The SIMS measurement shows that the orientation inversion regions J(defect accumulating regions H) on the masks have a higher carbonconcentration of about 1×10 ¹⁷ cm⁻³.

The low defect density single crystal regions Z have a lower carbonconcentration of 5×10¹⁶ cm⁻³ in Embodiment 4. The C-plane growth regionY has a still higher carbon concentration of 4×10¹⁸ cm⁻³. It isconfirmed that carbon is really doped into the gallium nitride crystal.It turns out that the carbon doping efficiency has a strong dependenceupon the growing planes. Carbon concentrations in Zs, Hs and Y satisfyan inequality Z<H<Y in Embodiment 4.

Further many experiments have been repeated. Examinations of results ofthe experiments clarify that carbon concentrations in the inversionregions J (defect accumulating regions H) are less than 10¹⁸ cm⁻³(H<10¹⁸ cm⁻³). The low defect single crystal regions Z, which have beengrown with maintaining facets, have carbon concentrations less than 10¹⁸cm⁻³ (Z<10¹⁸ cm⁻³). The C-plane growth region Y, which has been grownwith a flat C-plane, has a carbon concentration between 10¹⁶ cm⁻³ and10²⁰ cm⁻³ (10¹⁶cm⁻³≦Y≦10²⁰ cm⁻³). The C-plane growth region Y has thehighest carbon concentration among Y, Zs and Hs. Carbon ratios Y/H andY/Z are 10¹ to 10₅ (10_(1 ≦Y/H≦)10⁵, 10¹≦Y/Z ≦10₅). The carbon ratio Y/Hmeans a quotient of the carbon concentration of the C-plane region Ydivided by the carbon concentration in the defect accumulating regionsH. The carbon ratio Y/Z means a quotient of the carbon concentration ofthe C-plane region Y divided by the carbon concentration in the lowdefect single crystal regions Z.

The C-plane growth regions Y have the highest carbon concentrations. Butelectric conductivity is the lowest in the C-plane growth regions Yamong Ys, Hs and Zs. It is assumed that carbon does not generate n-typecarriers (free electrons) as an n-type dopant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view of a facet pit appearing on a growingsurface at a starting stage of growth for demonstrating the facet growthmethod proposed (2) by Japanese Patent Laying Open No.2001-102307 whichmakes hexagonal facet pits on a growing surface, grows GaN withoutburying the facet pits, concentrates dislocations at boundaries of thefacets and sweeps dislocations into bottoms of the facet pits.

FIG. 1(b) is a perspective view of a facet pit appearing on a growingsurface at a later stage of growth for demonstrating the facet growthmethod proposed by (2) Japanese Patent Laying Open No.2001-102307 whichmakes hexagonal facet pits on a growing surface, grows GaN withoutburying the facet pits, concentrates dislocations at boundaries of thefacets and sweeps dislocations into bottoms of the facet pits.

FIG. 2 is a plan view of a facet pit appearing on a growing surface fordemonstrating the facet growth method proposed by (2) Japanese PatentLaying Open No.2001-102307 which makes hexagonal facet pits on a growingsurface, grows GaN without burying the facet pits, concentratesdislocations at boundaries of the facets and gathers dislocations atbottoms of the facet pits.

FIG. 3(1) is a sectional view of a facet pit appearing on a growingsurface for demonstrating the facet growth method proposed by (2)Japanese Patent Laying Open No.2001-102307 which makes hexagonal facetpits on a growing surface, grows GaN without burying the facet pits,concentrates dislocations at boundaries of the facets, gathersdislocations at bottoms of the facet pits and makes a dislocationbundle. FIG. 3(2) shows a sectional view of a facet pit with a hazydispersion of dislocations escaping from the pit.

FIG. 4(1) is a sectional view of a pit or V-groove at an early stage fordemonstrating the mask-facet growth method proposed by (3) JapanesePatent Laying Open No. 2003-165799 and (4) Japanese Patent Laying OpenNo. 2003-183100 which make dislocation-attractive defect accumulatingregions (H) on masked parts, make low defect density single crystalregions (Z) under facets, produce C-plane growth regions (Y) underC-plane surfaces and maintain dislocations in the defect accumulatingregions (H). FIG. 4(2) shows a sectional view of the pit or V-grooves ata later stage for showing dislocations being arrested in the defectaccumulating regions (H) without being dispersed till the end of thegrowth.

FIG. 5(1) is a section of an undersubstrate (U) and a mask (M) formed onthe undersubstrate (U) at a start of the facet growth method. FIG. 5(2)is a section of the undersubstrate (U), the mask (M) and GaN crystals ata later stage for showing the masked parts prohibiting GaN growth,slanting facets starting from ends of the mask and rising to the C-planesurface. FIG. 5(3) is a section of the undersubstrate (U), the mask (M)and GaN crystals at a further later stage for showing the occurrence ofa defect accumulating region (H) on the masked parts and appearance oftwo step facets in the pit or the valley.

FIG. 6(1) is a perspective view of a prism-roofed GaN crystal producedby a facet growth method of forming mask stripes on an undersubstrate(U), growing GaN in vapor phase, producing facet valleys and makingdefect accumulating regions (H) on the stripe-covered parts. FIG. 6(2)is a perspective view of a pit-roofed GaN crystal produced by a facetgrowth method of forming mask dots on an undersubstrate (U), growing GaNin vapor phase, producing facet facet pits and making defectaccumulating regions on the dot-covered parts.

FIG. 7(1) is a section of an undersubstrate (U) and a mask (M). FIG.7(2) is a section of the undersubstrate (U), the mask (M) and GaNcrystals grown on exposed parts, and facets (F) starting from sides ofthe mask. FIG. 7.(3) is a sectional view for showing beaks (Q) appearingon slants of the facets. FIG. 7(4) is a sectional view for showing thebeaks (Q) meeting together above the mask and being unified. FIG. 7(5)is a section for showing GaN crystals growing on the unified beaks withthe same orientation as the beaks (Q).

FIG. 8(1) is a plan view of an undersubstrate (U) and linear parallelmask stripes (M) formed at a pitch p on the undersubstrate. FIG. 8(2) isa CL (cathode luminescence) image of a facet-grown, sliced and polishedGaN crystal having parallel low dislocation single crystal regions (Z),C-plane growth regions (Y) and defect accumulating regions (H).

FIG. 9(1) is a section of an undersubstrate (U). FIG. 9(2) is a sectionof the undersubstrate (U) and mask stripes (M) in the strip-mask facetgrowth. FIG. 9(3) is a section of the undersubstrate (U), the maskstripes (M) and a thick grown GaN crystal on the masked undersubstratewith defect accumulating regions (H) on the mask stripes, low defectsingle crystal regions (Z) below facets on exposed parts and C-growthregions (Y) below C-plane surfaces on the expose parts. FIG. 9(4) is aCL image of a flat HZYZHZYZH structured GaN crystal produced byeliminating the undersubstrate from the GaN crystal, grinding theseparated GaN crystal and polishing the GaN crystal. FIG. 9(5) is a CLimage of a flat HZHZH structured GaN crystal without C-plane growthregions (Y).

FIG. 10(1) is a plan view of an undersubstrate (U) and isolated maskdots formed at a pit p with six-fold symmetry on the undersubstrate (U)in the dot-mask facet growth. FIG. 10(2) is a CL image of a flat HZYhexagonal symmetric GaN crystal produced by eliminating theundersubstrate from the GaN crystal, grinding the separated GaN crystaland polishing the GaN crystal.

1. A method of growing a gallium nitride crystal comprising the stepsof: preparing an undersubstrate; forming masks which suppresses galliumnitride from epitaxially growing on the undersubstrate; making maskedparts and exposed parts on the undersubstrate; placing the maskedundersubstrate in a reaction furnace; growing gallium nitride on themasked undersubstrate in vapor phase; making pits of facets or groovesof facets with bottoms which correspond to the masks; doping growinggallium nitride with carbon for 0.5 hour to 2 hours at least at aninitial stage of growth; growing epitaxially low defect density singlecrystal regions Z having a polarity under the facets on the exposedparts of the undersubstrate till an end of growth; growing epitaxiallyC-plane growth regions Y under C-planes having the same polarity on theexposed parts of the undersubstrate till the end of growth; inducingbeaks on the facets having a polarity different by 180 degrees from thepolarity of the gallium nitride single crystals Z and Y on the exposedparts by carbon doping; and maintaining the facet pits or facet groovestill the end of growth.
 2. A method of growing a gallium nitride crystalcomprising the steps of: preparing an undersubstrate; forming maskswhich suppresses gallium nitride from epitaxially growing on theundersubstrate; making masked parts and exposed parts on theundersubstrate; placing the masked undersubstrate in a reaction furnace;growing gallium nitride on the masked undersubstrate in vapor phase;making pits of facets or grooves of facets with bottoms which correspondto the masks; doping growing gallium nitride with carbon for 0.5 hour to2 hours at least at an initial stage of growth; growing epitaxially lowdefect density single crystal regions Z having a polarity under thefacets on the exposed parts of the undersubstrate till an end of growth;growing epitaxially C-plane growth regions Y under C-planes having thesame polarity on the exposed parts of the undersubstrate till the end ofgrowth; inducing beaks on the facets having a polarity different by 180degrees from the polarity of the gallium nitride single crystals Z and Yon the exposed parts by carbon doping; expanding the beaks from thefacets above the masks; unifying the beaks into bridges having apolarity different by 180 degrees from the polarity of the galliumnitride single crystals Z and Y on the exposed parts; making polarityinversion gallium nitride single crystal regions J on the bridges abovethe masks; covering the masks the with the polarity inversion galliumnitride single crystal regions J having a polarity different by 180degrees from the polarity of the gallium nitride single crystals Z and Yon the exposed parts; and maintaining the facet pits or facet groovestill the end of growth.
 3. The method as claimed in claim 2, wherein the<0001> direction of the single crystal on the exposed parts is equal to<000-1> direction ofthe polarity inversion gallium nitride singlecrystals on the masks.
 4. The method as claimed in claim 2, whereingallium nitride buffer layers with a thickness less than 200 nm aregrown on the undersubstrate with the masks at a low temperature between400° C. and 600° C. before epitaxially growing gallium nitride singlecrystals.
 5. The method as claimed in claim 4, wherein the temperatureof growing epitaxially gallium nitride single crystals is 900° C. to1100° C.
 6. The method as claimed in claim 2, wherein the undersubstrateis a sapphire wafer, a Si wafer, a SiC wafer, a GaN wafer, a GaAs waferor a foreign material wafer coated with a GaN thin layer.
 7. The methodas claimed in claim 2, wherein the growth in vapor phase is HVPE(hydride vapor phase epitaxy).
 8. The method as claimed in claim 2,wherein a hydrocarbon gas is supplied into the reaction furnace fordoping growing gallium nitride with carbon.
 9. The method as claimed inclaim 8, wherein the hydrocarbon gas is CH₄, C₂H₆ or C₂H_(4.)
 10. Themethod as claimed in claim 8, wherein the hydrocarbon gas has a partialpressure from 1×10⁻⁴ atm (10 Pa) to 5×10⁻² atm (5 kPa).
 11. The methodas claimed in claim 2, wherein solid carbon is laid in the reactionfurnace for inducing a reaction of carbon with HCl gas, synthesizinghydrocarbon gases and doping growing gallium nitride with carbon. 12.The method as claimed in claim 11, wherein the synthesized hydrocarbonhas a partial pressure from 1×10⁻⁴ atm (10 Pa) to 5×10⁻² atm (5 kPa).13. A gallium nitride substrate comprising: low defect density singlecrystal regions Z being grown under facets on exposed parts in a facetgrowth method and having carbon concentration less than 10 ¹⁸ cm⁻³;polarization inversion regions J being grown on masks in the facetgrowth method and having a carbon concentration less than 10¹⁸ cm⁻³;C-plane growth regions Y being grown under C-planes on exposed parts andhaving a carbon concentration of 10¹⁶ cm⁻³ to 10²⁰ cm⁻³; and carbonconcentration ratios Y/J and Y/Z being 10¹ to 10⁵.
 14. The galliumnitride substrate as claimed in claim 13, wherein the low defect densitysingle crystal regions Z have grown under {11-22} facets.
 15. Thegallium nitride substrate as claimed in claim 13, wherein the polarityinversion regions J have grown under {11-2-6} facets.